This application is based on Application No. 2001-45496, filed in Japan on Feb. 21, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) system for an internal combustion engine, and more specifically, to improvements in such an EGR valve control apparatus.
2. Description of the Related Art
Conventionally, in automotive engines, etc., there has been provided an exhaust gas recirculation system, so-called EGR (exhaust gas recirculation) apparatus, for recirculating exhaust gas into an air fuel mixture sucked into an internal combustion engine in order to reduce the generation of NOx and the like which are harmful components in the exhaust gas.
However, recirculation of an improper amount of exhaust gas results in reduction in the engine output power and instability in the combustion (decreased engine rotation, tardiness, etc.), causing an deterioration in driveability. Thus, in order to reduce these problems, it is necessary to control the amount of exhaust gas to be recirculated (EGR amount) according to the operating conditions.
In this case, to control the EGR amount in accordance with the operating conditions of the engine, a control valve (EGR valve) is provided on an exhaust gas recirculation passage connecting between an intake passage and an exhaust passage of the engine.
An exhaust gas recirculation apparatus is known which includes a step motor as actuator for driving such an EGR valve. Target values for the EGR amount are determined in advance in accordance with the engine operating conditions so as to properly balance the output performance and the exhaust performance of the engine with each other. A target EGR amount suited to the current engine operating conditions is determined by referring to a map of such target values. This target EGR amount is then converted into a corresponding control command value, which is in turn output to the step motor, so that the amount of driving of the step motor immediately follows the target EGR amount (the control command value).
The target EGR value is determined based on the charging efficiency and the number of revolutions per unit time of the engine. The rotational angle of the step motor changes in a stepwise manner or in a plurality of steps, but if the rotational angle of the step motor is always controlled to follow the number of revolutions per unit time of the engine, etc., there might be caused frequent opening and closing of the EGR valve, so such control is disadvantageous in respect of durability. To suppress this phenomenon, in the past, control with hysteresis is carried out in such a manner that the position of the step motor is not changed unless a difference between the target position and the current position of the step motor is two steps or greater (i.e., with hysteresis of one step).
In such control by the known EGR valve control apparatus, however, the current position of the step motor remains deviated from the target position thereof under the hysteresis control as long as the difference between the calculated target position and the detected current position of the step motor is less than two steps (hysteresis of one step), thus posing a problem from a point of view of accurate control.
In addition, the hysteresis of one step in the known hysteresis control is not necessarily satisfactory, and hence the EGR valve might be frequently caused to open and close due to a change in the number of revolutions per unit time of the engine, etc., thus posing another problem from a point of view of durability.
Further, although the target position of the step motor is determined based on the number of revolutions per unit time of the engine (engine rotation) and the charging efficiency, there might take place a phenomenon that the recirculation rate of exhaust gas (EGR rate) becomes too high upon rapid acceleration, thereby providing problems of reduction in the engine rotational speed and tardiness in operation.
The present invention is intended to obviate the above-mentioned problems, and has for its object to provide an EGR valve control apparatus which is capable of controlling an EGR amount in an accurate and proper manner by using a step motor, thus improving the durability of an EGR valve.
In view of the above object, the present invention resides in an EGR valve control apparatus in which an opening of an EGR valve for opening and closing an exhaust gas recirculation passage connecting between an exhaust passage and an intake passage is adjusted discretely in a plurality of steps, and which comprises: a detecting section for detecting operating conditions of an internal combustion engine; a step motor target position calculating section for calculating a target position of a step motor in accordance with the operating conditions of the internal combustion engine detected by the detecting section; a difference calculating section for calculating a difference between a current position and the target position of the step motor; a hysteresis-provided drive control section for driving the step motor in accordance with the target position and performing control with hysteresis of two steps or more so as to hold the position of the step motor when a difference between the target position and the current position of the step motor is equal to or less than a prescribed n steps which is equal to or greater than two steps; and a forced drive control section for driving the step motor to the target position thereof while stopping the hysteresis-provided control when a deviation between the target position and the current position of the step motor remains for a predetermined period of time.
Preferably, the EGR valve control apparatus further comprises: a low speed operation determining section for determining whether a vehicle, on which the EGR valve control apparatus is installed, is at a low speed which is less than a prescribed low speed determination threshold; a rapid acceleration determining section for determining whether the vehicle is in a rapid acceleration state in which an acceleration of the vehicle is greater than a prescribed rapid acceleration determination threshold; and a follow-up speed slow-down drive control section for slowing down a follow-up speed of the step motor to follow the target position thereof which is calculated by the step motor target position calculating section in accordance with the operating conditions of the internal combustion engine when it is determined that the vehicle is in the rapid acceleration state during travelling at the low speed.
Preferably, the hysteresis-provided drive control section performs driving control with hysteresis of two steps so as to hold the position of the step motor when the difference between the target position and the current position of the step motor is equal to or less than two steps.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.